1. Field of the Invention
The present invention relates to a communication apparatus, and more particularly, to a communication apparatus that carries out radio communications combining a CDMA (Code Division Multiple Access) system and OFDM (Orthogonal Frequency Division Multiplexing) system in mobile communications.
2. Description of the Related Art
An error rate characteristic in a communication based on a CDMA system deteriorates in a multi-path environment because of interference between spreading codes. On the other hand, a well-known communication system resistant to interference between codes is an OFDM communication that uses a guard interval. Thus, a radio communication based on an OFDM-CDMA system that implements a CDMA-based communication with multiple carriers and performs transmission with subcarriers assigned to their respective chips then subjected to frequency division multiplexing is now a focus of attention as a next-generation radio communication system.
In an OFDM-CDMA-based communication, a plurality of signals is spread using mutually not correlated spreading codes by assigning one spread signal to one subcarrier. If these spreading codes are completely orthogonal to each other, signals other than the necessary ones are completely removed through despreading processing at the time of reception regardless of the degree of signal multiplexing.
Hereinafter, a conventional OFDM-CDMA-based communication apparatus will be explained using FIG. 1. FIG. 1 is a block diagram showing a configuration of a conventional OFDM-CDMA-based communication apparatus.
In the transmission system shown in FIG. 1, each spreading section 11 carries out spreading processing by multiplying transmission signals 1 to n by their respective spreading codes 1 to n. Here, suppose their spreading factor is k.
Addition section 12 adds up the transmission signals subjected to spreading processing. Serial/parallel (hereinafter referred to as “S/P”) converter 13 converts a serial signal to a plurality of parallel signals. This S/P converter 13 divides the transmission signals thus spread and added up by spread signal or breaks down spread transmission signals 1 to n by spread signal (chip), that is, A 1st to kth chip.
IFFT processing section 14 carries out inverse Fourier transform processing on a plurality of parallel signals. This IFFT processing section 14 assigns one subcarrier to one chip data signal string and carries out frequency division multiplexing.
That is, the number of subcarriers corresponds to the spreading factor and it is “k” in this case. Suppose the 1st chip of transmission signals 1 to n is placed in subcarrier 1 and the kth chip of transmission signals 1 to n is placed in subcarrier k. That is, a chip data string is subjected to frequency division multiplexing. FIG. 2 shows this mode. Antenna 15 transmits/receives a radio signal.
In the reception system, quasi-coherent detection section 16 carries out quasi-coherent detection processing on the reception signal from antenna 15. That is, quasi-coherent detection section 16 carries out quasi-coherent detection processing under the control of a local signal subjected to frequency offset correction from frequency offset correction section 17, which will be described later. In this way, frequency offset correction is performed.
Frequency offset correction section 17 detects a frequency offset using the signal after quasi-coherent detection processing and creates a local signal based on this frequency offset. That is, frequency offset correction section 17 outputs the local signal subjected to frequency offset correction to quasi-coherent detection section 16.
FFT processing section 18 carries out Fourier transform processing on the reception signal subjected to quasi-coherent detection processing and extracts each subcarrier signal (chip data signal string). Transmission path compensation sections 19 are provided in one-to-one correspondence with subcarriers and carry out compensation processing such as phase compensation on their respective subcarrier reception signals.
Parallel/serial (hereinafter referred to as “P/S”) converter 20 converts a plurality of parallel signals into a single serial signal. This P/S converter 20 rearranges the subcarrier signals from one chip to another and outputs the 1st chip of a signal on which spread transmission signals 1 to n are multiplexed at time t1, the 2nd chip of a signal on which spread transmission signals 1 to n are multiplexed at time t2, . . . up to the kth chip of a signal on which spread transmission signals 1 to n are multiplexed at time tk.
Despreading sections 21 carry out despreading processing by multiplying the reception signal which has been converted to a single serial signal by their respective spreading codes 1 to n and extracting only the signals spread using those codes.
However, the above OFDM-CDMA-based communication apparatus has problems as shown below. That is, if the frequency offset detected by frequency offset correction section 17 above contains a detection error, the reception signal after FFT processing contains a residual phase error.
This results in the reception signal after FFT processing involving phase rotation. For example, as shown in FIG. 3, if the frequency offset contains a detection error of Δ f, the 1st chip to kth chip corresponding to 2nd transmission signals 1 to n contain a residual phase error with 2π Δ fT. The 1st chip to kth chip corresponding to 3rd transmission signals 1 to n contain a residual phase error with 2π Δ f2T. Here, T is signal transmission speed before spreading processing.
Thus, the reception signals obtained from those signals containing residual phase errors have a deteriorated error rate characteristic.